I don't think Rossi, in his mind, has anything to prove on the Ducati at the end of this season. He gave it 2 years, and apparently they couldn't improve the bike enough for him to be competitive (like I'm sure he hoped in the beginning).
Of course, next year, on the Yamaha, we will find out if he still IS competetive.
"The atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts."
- Werner Heisenburg